The present invention relates to a novel method for preparing improved polyetherimide-polyimide copolymers. The novel method results in polyetherimide-polyimide copolymers that have superior properties, including high glass transition temperatures, a completely transparent appearance with no insolubles present and no visible phase separation. The copolymers prepared by this method are flexible and can be extruded using conventional extrusion methods and apparatus without the hazard of the loss of volatile, and potentially toxic organic diamines.
Polyetherimides are well-known, high performance engineering thermoplastics that are typically prepared by reacting a bis(ether anhydride) with an organic diamine. Polyetherimide-polyimide copolymers, in which a portion of the bis(ether anhydride) is replaced by a more rigid dianhydride, such as pyromellitic anhydride, have been described in the literature. In general, these copolymers have higher glass transition temperatures (Tg) and heat distortion temperatures (HDT) than the corresponding polyetherimides, while maintaining their physical properties.
There are various methods currently employed in the preparation of polyetherimides. U.S. Pat. No. 4,417,044, discloses a method for preparing polyetherimides from an aromatic bis(ether anhydride) with an organic diamine in an inert solvent.
U.S. Pat. Nos. 3,983,093, and 4,565,858, disclose polyetherimides prepared from bisphenol dianhydride, organic diamines and pyromellitic dianhydride. The methods described in these patents involve reacting a mixture of a bis(ether anhydride) and another anhydride, such as pyromellitic anhydride, with an organic diamine. Other methods for preparing polyetherimides can be found in U.S. Pat. Nos. 4,794,157, 4,965,337, and 4,612,361.
Efforts to enhance the physical properties of a polyetherimide-polyimide copolymer prepared from a bis(ether anhydride) and an organic diamine by substituting pyromellitic dianhydride for 10 mole % or more of the bis(ether anhydride) have met with significant difficulties. The resulting copolymer often has poor surface appearance (pitting), is opaque with discrete phase separation, is very brittle, and generally is unsuitable for most commercial purposes, i.e., films, molding compounds, and coatings. It is believed that these characteristics are the result of the formation of polyimide blocks that are not soluble in the copolymer matrix.
Accordingly, a need exists for a process for preparing polyetherimide-polyimide copolymers having improved glass transition temperatures, improved physical appearance, homogeniety in composition, flexibility, and without the emission of potentially toxic organic amines during production.